1. Toss salmon pieces in a bowl with soy sauce, toasted sesame oil, garlic powder and paprika. Let marinate for at least 30 minutes (I left mine in fridge for a couple hours).
2. Place rice in a fine-mesh strainer and rinse well with cold water until it runs clear. Drain and transfer to a saucepan.
3. Add water to the rice and briefly stir. If you have time, you can leave the rice to soak for 20-30 minutes (this step is optional, but it helps the rice to cook more evenly).
4. Preheat oven to 400F (200C).
5. Make sushi vinegar. In a saucepan, lightly heat rice vinegar, maple syrup and salt. Stir until salt is dissolved and set aside.
6. Bring the saucepan with rice to a simmer and cook covered for 15 minutes. Turn off the heat, add sushi vinegar, gently stir, cover and let stand for another 10 minutes.
7. Place 1 heaping Tablespoon of rice to the middle of nori squares. Transfer each nori square to a muffin cup, pushing down slightly so that the rice is at the bottom.
8. Add 3-4 salmon pieces on top of the rice.
Transfer muffin cups to a baking sheet and bake in the preheated oven for about 8 minutes (or more, until you reach the desired doneness).
9. Meanwhile, make spicy mayo. In a cup, combine avocado mayo, sriracha, sesame oil, lime juice and water (optional, use water only if your mayo is thick).
Transfer to a plastic bag (cut one tip off, so you can squeeze a thin drizzle).
10. Top salmon sushi bites with a drizzle of mayo, sesame seeds, diced avocado and scallions. Enjoy!